I can only think of two causes. A filter that moves mail from that domain to the trash. (lots of folk have these manual filter that they try and manage spam with. Your mail provider is yahoo and they have decided that the domain is a spammer and they keep putting the mail in there. A third option is you have an add-on from one of the antivirus vendors that has dropped the ball, this generally happens after Thunderbird updates because the antivirus companies do not follow the rules for add-ons.